Optical Phase-shift keyed (PSK) modulation formats have great utility for optical communications because of their suitability for high-speed long-haul transmission, with good spectral and photon efficiency. Good photon efficiency is important for free space communications applications which may often be limited by channel loss rather than, for example, dispersive properties of a guided channel. PSK modulation formats are also suitable for use with coherent receivers that may achieve even higher efficiency and mitigate dispersive channel effects. Commercial systems incorporating PSK modulation are being deployed to support data rates at 10 Gbps and beyond.
For communications systems that operate over dynamically changing channel conditions, it is useful to support operations over a range of data rates in order to provide features such as fall-back modes of operation and bandwidth on-demand, which may help increase overall channel throughput.
Differentially encoded PSK (DPSK) receivers commonly use interferometric demodulators, such as optical delay-line interferometers (DIs), to perform a phase comparison between differentially encoded symbols. Typically, the DI delay τd is equal to one symbol period τs or an integer multiple of the symbol period nτs, in which the phase comparison is performed on non-adjacent symbols separated by multiple symbols. The use of non-adjacent (NA) demodulation techniques enables transmitter and receiver implementations that may operate a multiple rates that are integer submultiples of the DI delay, or, equivalently, rates that are integer multiples of the DI free spectral range (FSR), where FSR=1/τs, Rate(n)=nFSR=n/τd, and n is an integer rate multiplier. The FSRs of commercially available DIs range from ˜2 GHz to many tens of GHz (and beyond). Since larger FSRs correspond to smaller DI delays, DI packaging is often simplified and size-reduced with increasing FSR. In addition, smaller size DIs are typically easier to stabilize, which is beneficial in communication applications where the DI alignment and stability have a big impact on receiver sensitivity. Using commercially available DIs, multi-rate NA-DPSK demodulation techniques may operate at rates above the few Gbps regime, e.g, 2 Gbps, 4 Gbps, . . . , 10 Gbps, and beyond. Alternative multi-rate DPSK reception techniques include the use of multiple interferometers, each for a different data rate, though this approach is unattractive from a standpoint of size, cost, and complexity.